1. Field of Invention
This invention relates to a photo diode array; and more particularly, to such photo diode array having improved linearity.
2. Description of the Prior Art
A photo diode is formed, for example, by providing a PN junction with a p-type region on the light receiving surface side and an n-type region on the substrate side. A current generated by light applied to the photosensitive area flows from a cathode to an anode.
FIG. 1 is a sectional view of an exemplary conventional photo diode, wherein a p-type diffused region 2 is formed on one side of n-type layer 1 which is a substrate; n-contact 5 is formed on the opposite side of n-type layer 1; and passivation layer 3 is formed on the light receiving surface side, where p-type diffused region 2 is formed. A p-contact 4, connected to p-type diffused region 2, is formed by removing part of passivation layer 3 using an etching process, or the like.
The photo diode of FIG. 1 is operated as follows. Electrons, generated in the photo diode, by application of input light 100 to the photo sensitive area, are accumulated in n-type layer 1, while holes are accumulated in p-type diffused region 2. Accordingly, n-type layer 1 is negatively charged and p-type diffused region 2 is positively charged. Thus, when an external circuit is connected between p-type diffused region 2 and n-type layer 1, electrons and holes flow toward opposite layers from n-type layer 1 and p-type diffused region 2, respectively.
For example, an electron generated in p-type diffused region 2, shown as xe2x80x9cEL01xe2x80x9d in FIG. 1 is accelerated towards n-type layer 1 by the electric field in the photo diode and accumulated there. In a similar manner, a hole generated in n-type layer 1, shown as xe2x80x9cHL01xe2x80x9d, is accelerated towards p-type diffused region 2 and accumulated there. A photo diode array may comprise a plurality of such photo diodes arranged in an array.
FIG. 2 shows an example of a configuration of a plurality of photo diodes arranged in an array, wherein a plurality of photo diodes 6a-6e are arranged in the array on a substrate (not shown, for sake of convenience, but understood to be present. Same comments apply with the remainder of the drawing, namely, FIGS. 3-5) so that the photo sensitive areas thereof are oriented in a particular direction and electrodes 7a-7e are connected to one end of the photo sensitive area of each photo diode 6a-6e, respectively.
Operation of the FIG. 2 array is as follows. When light is made incident to the photo sensitive area of photo diodes 6a-6e, currents are generated between each light receiving surface side and each opposite side. The currents are outputted to the outside via electrodes 7a-7e, respectively. For example, holes generated in photo diodes 6a-6e are outputted via electrodes 7a-7e. 
However, disadvantageously, in conventional arrays, current values outputted thereby may vary according to the location on the photo sensitive area at which the input light is applied. For example, when the input light is made incident to a portion-marked xe2x80x9cP001xe2x80x9d, the distance from that spot to electrode 7e is longer than when the input light is made incident to the portion marked xe2x80x9cP002xe2x80x9d. This causes a voltage drop due to the resistance that exists between the respective portions and the electrode. Hence, the current value which is outputted can vary. In other words, a problem exists in that in the prior art, linearity deteriorates depending on the location on the light sensitive area at which the input light is made incident. Specifically, when a high current is generated by the light being made incident to the spot marked xe2x80x9cP001xe2x80x9d, linearity deterioration increases, as compared to the light being made incident to spot xe2x80x9cP002xe2x80x9d,because the generated voltage drop increases further.
In order to reduce this linearity deterioration, a bias voltage which forms a reverse bias is generally applied to the photo diode. However, although this technique may improve frequency response and some amount of the linearity, in such a case, there is still the problem that this causes an increase in dark current which increases noise, and increases danger of destruction of the photo diode.
Accordingly, an object of the invention is to overcome the foregoing and other deficiencies and disadvantages of the prior art.
Another object is to realize a photo diode array wherein linearity is greatly improved.